Activation pathways of 5-fluorouracil in rat organs and in PC12 cells

Activation of the pyrimidine analogue 5-fluorouracil (5-FU) to the ribonucleotide level may occur through one of the following three pathways: 1) the 5-phosphoribosyl 1-pyrophosphate (PRPP)-mediated direct transfer of ribose 5-phosphate to 5-FU as catalysed by orotate phosphoribosyltransferase; 2) the ribose 1-phosphate (Rib1-P)-mediated addition of ribose by uridine phosphorylase, followed by the action of uridine kinase; and 3) the 2'-deoxyribose 1-phosphate (deoxyRib1-P)-mediated addition of deoxyribose, thought to be catalysed by thymidine phosphorylase, followed by the action of thymidine kinase. Many of the conclusions as to the precise pathways by which normal tissues and different cell lines activate uracil are indirectly derived from drug interactions affecting the availability of the substrates of the three pathways, or from measurement of activities of the enzymes metabolising 5-FU in normal tissues and tumours. In previous papers (Cappiello et al. Biochim Biophys Acta 1998;1425:273--81; Mascia et al. Biochim Biophys Acta 1999;1472:93--8), we assessed the molecular mechanisms by which the natural base uracil is salvaged in vitro to uracil ribonucleotides and deoxyribonucleotides in rat liver and brain. In this paper, we investigated the pathways of 5-FU activation to cytotoxic ribonucleotide and deoxyribonucleotide levels in normal rat tissues and PC12 cell extracts. The results clearly showed that normal rat tissues activated 5-FU mainly via the Rib1-P pathway, and to a lesser extent via the PRPP pathway. The deoxyRib1-P pathway was absent. PC12 cells activated 5-FU mainly via the PRPP pathway and to a lesser extent by the other two pathways.